Study of the feasibility of
investigating any potential relationship between the supply of
discoloured water and gastrointestinal illness
DWI0842
November 2005
Executive Summary
Background
It is not clear whether events resulting in a sudden obvious
discolouration of the water supply could result in an increase in the
risk of gastrointestinal illness among those exposed. The reviewers
were commissioned by the Drinking Water Inspectorate to evaluate the
available evidence relevant to the potential association between
drinking water discolouration at point of use and risk of acute
gastrointestinal illness. The purpose of this review is to assess the
existing evidence, to make recommendations regarding the need for a
further study in England and Wales, and to inform the design of such a
study.
Questions
Addressed by the Review This review sought to find
evidence relevant to a UK setting of an association between discoloured
tap water and the risk of acute gastrointestinal illness and to
determine what proportion of acute gastrointestinal illness in England
and Wales might be attributable to exposure to discoloured water.
Review
Methods The reviewers identified peer reviewed research
from structured searches of databases, and non-peer reviewed research
from relevant websites. Studies were included in the review if they
addressed the question of the association between discolouration of a
public water supply in a setting relevant to the United Kingdom and
risk of acute gastrointestinal illness. Eligible
papers were assessed independently by two reviewers in terms of
appropriateness of the study design and analysis, results,
interpretation, and the overall quality of the paper.
Results
of the Review Time-series studies investigating the effect
of temporal variations in turbidity at the treatment plant and
subsequent incidence in acute gastrointestinal illness were the only
category of studies reviewed deemed sufficiently rigorous to be
informative; in general these studies provided evidence of an increase
in acute gastrointestinal illness at varying lags following days of
high turbidity (range 3 to 29 days). One study investigating the effect
of turbidity at point of use was identified, but was judged to be
insufficiently rigorous to be informative.
Discussion
No studies were identified in this review that specifically addressed
the issue of water discolouration incidents and their effect on the
risk of acute gastrointestinal illness. With the exception of
contamination with highly toxic metals, we have found no evidence that
water discolouration at point of use is associated with acute
gastrointestinal illness. Only one poor-quality study identified in
this review measured water quality at point of use and this study found
no association. Most studies identified in this review investigated the
effect of variations in turbidity within normal limits at point of
treatment or water treatment works final water. These studies address a
fundamentally different hypothesis and their results cannot be used to
assess the likely effect of water discolouration at point of use. In
order to establish whether there is an association between
discolouration events and risk of acute gastrointestinal illness in the
United Kingdom setting, an epidemiological study specifically designed
to address this question would be needed.
Conclusions
There is some evidence that increases in turbidity of final water are
associated with subsequent increases in the incidence of acute
gastrointestinal illness at varying lags. Lags of between 4 and 13 days
were commonly reported. A peak in acute gastrointestinal illness at
certain lags following days of high final water turbidity was
consistently found across studies. This association is unlikely to be
the result of measurement error, bias or random error and warrants
further investigation. The potential for residual confounding (due to
inadequate adjustment for time-varying confounding factors such as
seasonal effects, temperature and precipitation) in these studies
remains unclear. Further methodological work in this area could clarify
this issue. No evidence was found as to whether turbidity levels at
point of use- in the absence of increased turbidity at the treatment
plant- increase the risk of acute gastrointestinal illness.
Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.